Ask Wen?
Find the direct object.

One question. One answer. That answer is always Akkusativ. The direct object has a question word built for it. Use it and you will never confuse subject and object again.

Related: The German Akkusativ Case, the full picture of the direct object. And the pronouns that answer wen? every time: Personal pronouns. The Akkusativ set.

Wen? means: who is receiving the action?

Wen means "whom." You point it at the verb and ask: who is the action landing on? The word that answers is your direct object. It is Akkusativ. In edupango, it is always green.

Three sentences. Same question every time:

• DerHund beißtdenMann. Wen beißt er? → denMann.
• Sie ruftihren Bruder. Wen ruft sie? → ihren Bruder.
• Ich kennediesen Mann. Wen kenne ich? → diesen Mann.

In every case, the answer is green. The action leaves the subject and lands on it.

Was? is Wen? for things

Wen? finds people. Was? finds things. Together they cover every direct object in the language. The case is the same → Akkusativ.

• Er liestdasBuch. Was liest er? → dasBuch.
• Sie trinkteinenKaffee. Was trinkt sie? → einenKaffee.

If you are not sure which to ask, ask both. Only one will make sense. That is the one.

The one change you actually need to know

The Akkusativ looks exactly like the Nominativ except for one case: masculine singular. Only there does the article change.

Gender	Nominativ	Akkusativ
Masculine	derMann	denMann
Feminine	dieFrau	dieFrau
Neuter	dasKind	dasKind
Plural	dieLeute	dieLeute

One change. One gender. That is the entire Akkusativ article system.